When fluororesins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymers (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA), etc. are coated on various metal substrates, it is difficult to apply the fluororesins directly on the metal substrates since the fluororesins are non-tacky as their nature.
Therefore, to coat the fluororesins on the substrates, primer compositions, which have adhesion properties to the substrates and also to the fluororesin coating to be applied on the primers, have been developed and practically used.
In recent years, engineering plastics having excellent heat resistance are developed and thus new primers for a fluororesin coating are being developed by combining such plastics and the fluororesins.
For example, JP-A-61-111352 discloses a fluororesin coating composition consisting of an aqueous dispersion of polyethersulfone and a fluororesin. However, when this composition is used as a primer, the corrosion resistance, which is the drawback of polyethersulfone, tends to further deteriorate.
JP-A-59-199774 discloses a primer composition comprising polyamideimide or polyimide as a binder component, and a specific fluororesin. However, this composition is severely discolored in the course of baking (calcination) so that the color is changed to dark brawn. Thus, the color tone of the coating is limited. In addition, since the composition contains the highly water-absorbing binder resin, the coating of the fluororesin tends to blister due to low steam resistance in the case of rice cookers, in which rice is quickly heated, such as IH cooking jars, when the primer composition is used as the primer for a PFA powder coating which is increasingly used as the coating of the rice cookers.
JP-B-2702041 discloses a primer composition comprising polyethersulfone, polyamideimide and/or polyimide, a fluororesin, and metal powder, which are dissolved or dispersed in an organic solvent. However, in the primer composition disclosed in this patent, the dispersion medium is the organic solvent such as N-methylpyrrolidone, and the binder resin is dissolved in the solvent. Therefore, it is difficult to increase the solid content of the coating. In addition, the composition of the dispersion medium has to be improved from the viewpoint of the environmental protection, which is one of the big social problems.